Tumor cell growth absolutely requires adequate supply of oxygen and nutrients. Hypoxia inducible factors (HIF), a family of heterodimeric transcription factors, play a key role in meeting this requirement. High levels of HIF-1alpha expression and abnormal angiogenesis have been detected in many types of solid tumors. Multiple non-hypoxic causes lead to HIF activation, including growth factors, loss of tumor suppressors, activation of oncogenes, and mitogenic signaling. The importance of HIF in apoptosis, cell proliferation, and tumor growth has also been demonstrated by genetic knockout studies. HIF promotes tumor growth through multiple mechanisms. First, HIF is directly responsible for the transcription of target genes including genes encoding vascular endothelial growth factor (VEGF), and its receptor, that have essential roles in angiogenesis. In addition, HIF activation modulates the energy and glucose metabolism in tumor cells, which may facilitate their growth and survival. So, it is plausible to hypothesize that inhibition of HIF activation will repress tumor-induced angiogenesis and vascularization, thus suppressing tumor growth and metastasis. Therefore, blocking HIF-1 activation is a potential treatment for cancer, especially for highly angiogenic tumors with high levels of HIF. Our long-term goal is to develop practical and efficient methods to modulate HIF activity. HIF activity is determined by both the protein stabilization and the functional stimulation of the alpha subunits. The stabilization of HIF protein is regulated by hydroxylation-dependent, pVHL (von Hippel Lindau)-mediated ubiquitination and subsequent degradation. The transactivation activity is governed by the interaction, physical and functional, between HIF-alphaCAD and p300/CBP. This proposal focuses on a better understanding of the functional activation of HIF-alpha. Specifically, the molecular mechanisms by which post-translational modifications affect the HIF and p300/CBP activity will be examined. Two common forms of modifications, acetylation and phosphorylation, which link various signals to gene expression and have been involved in HIF-activation, will be the major focus. In addition, the identified cellular factors that interact with an intramolecular inhibitory region of HIF-1alpha will be further characterized. Special efforts will be made to investigate whether these proteins are components of the enzymes responsible for posttranslational modifications, or they are the substrates of such modifications. The role of these proteins in tumor angiogenesis and tumorigenesis will be studied in mouse xenograft models.